The present application claims priority to Japanese Patent Application No. H11-204038 filed on Jul. 19, 1999, Japanese Patent Application No. P2000-058116 filed on Mar. 3, 2000, and Japanese Patent Application No. P2000-157509 filed on May 29, 2000. The above-referenced Japanese patent applications are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a proton conductor, a production method thereof, and an electrochemical device using the proton conductor.
2. Description of the Prior Art
In recent years, as a polymer solid-state electrolyte type fuel cell has been used to power cars, there has been known a fuel cell using a polymer material having a proton (hydrogen ionic) conductivity such as a perfluorosulfonate resin (for example, Nafion(copyright) produced by Du Pont).
As a relatively new proton conductor, there has also been known a polymolybdate having large amount of hydrated water such as H3Mo12PO40.29H2O or an oxide having a large amount of hydrated water such as Sb2O6.5.4H2O.
The above-described polymer material and hydrated compounds each exhibit, if placed in a wet state, a high proton conductivity at a temperature near ordinary temperature.
For example, the reason why the perfluorosulfonate resin can exhibit a very high proton conductivity even at ordinary temperature is that protons ionized from sulfonate groups of the resin are bonded (hydrogen-bonded) with moisture already entrapped in a polymer matrix in a large amount, to produce protonated water, that is oxonium ions (H3O+), and the protons in the form of the oxonium ions can smoothly migrate in the polymer matrix.
More recently, there has been also developed a proton conductor having a conduction mechanism quite different than that of each of the above-described proton conductors.
That is to say, it has been found that a composite metal oxide having a perovskite structure, such as, SrCeO3 doped with Yb, exhibits a proton conductivity without use of moisture as a migration medium. The conduction mechanism of this composite metal oxide has been considered such that protons are conducted while being singly channeled between oxygen ions forming a skeleton of the perovskite structure.
The conductive protons, however, are not originally present in the composite metal oxide but are produced by the following mechanism: namely, when the perovskite structure contacts the steam contained in an environmental atmospheric gas, water molecules at a high temperature react with oxygen deficient portions which have been formed in the perovskite structure by doping Yb or the like, to generate protons.
The above-described various proton conductors, however, have the following problems.
The matrix material such as the above-identified perfluorosulfonate resin must be continuously placed in a sufficiently wet state during use in order to keep a high proton conductivity.
Accordingly, a configuration of a system, such as, a fuel cell using such a matrix material, requires a humidifier and various accessories, thereby giving rise to problems in enlarging the scale of the system and raising the cost of the system.
The system using the matrix material has a further problem that the range of the operational temperature must be limited for preventing the freezing or boiling of the moisture contained in the matrix.
The composite metal oxide having the perovskite structure has a problem that the operational temperature must be kept at a high temperature of 500xc2x0 C. or more for ensuring an effective proton conductivity.
In this way, the related art proton conductors have the problems that the atmosphere dependence on the performance of each conductor becomes high, and more specifically, moisture or stream must be supplied to the conductor to ensure the performance of the conductor, and further, the operational temperature of the conductor is excessively high or the range of the operational temperature is limited.
A first object of the present invention is to provide a proton conductor which is usable in a wide temperature range including ordinary temperature and has a low atmosphere dependence, that is, it requires no moisture despite whether or not the moisture is a migration medium; to provide a method of producing the proton conductor; and to provide an electrochemical device that employs the proton conductor. To meet this objective, the proton conductor includes a proton conductor material that at least has number of functional groups so as to be capable of transferring hydrogen protons between the functional groups of the proton conductor material. The proton conductor material includes a wide variety of carbonaceous materials, examples of which are described in greater detail below with respect to the various illustrative embodiments of the present invention, such as, the first, second, third and fourth proton conductors, production methods and electrochemical devices thereof.
A second object of the present invention is to provide a proton conductor which exhibits a film formation ability while keeping the above-described performance, to be thereby usable as a thin film having a high strength, a gas permeation preventive or impermeable performance, and a good proton conductivity, to provide a method of producing the proton conductor, and to provide an electrochemical device using the proton conductor.
The present invention provides a first proton conductor mainly containing a fullerene derivative obtained by introducing a number of functional groups so as to be capable of transferring protons between the functional groups of the fullerene derivative. The fullerene derivative includes a fullerene molecule or a plurality of fullerene molecules that each contain the functional groups so as to provide the fullerene derivative with the desirable proton conductivity properties as discussed and will be discussed in greater detail below.
The present invention also provides a first method of producing a proton conductor, including the steps of: producing a fullerene derivative by introducing functional groups so as to be capable of transferring protons as previously discussed; and compacting a powder of the fullerene derivative into a desired shape.
The present invention also provides a first electrochemical device including: a first electrode, a second electrode, and a proton conductor held between the electrodes, wherein the proton conductor mainly contains a fullerene derivative as described above.
According to the first proton conductor of the present invention, since the conductor mainly contains the fullerene derivative having a proton transfer capability, protons are easily transferred or conducted, even in a dry state, and further, the protons can exhibit a high conductivity in a wide temperature range (at least in a range of about 160xc2x0 C. to xe2x88x9240xc2x0 C.) that includes ordinary temperatures. While the first proton conductor of the present invention has a sufficient proton conductivity even in a dry state, it can also have a proton conductivity in a wet state. The moisture may come from the outside.
According to the first production method of the present invention, since the production method includes the steps of: producing a fullerene derivative by introducing functional groups as discussed and molding a substance comprising the fullerene derivative, the proton conductor can be efficiently produced having the above-described unique performance without use of any binder resin. The term xe2x80x9cmoldingxe2x80x9d means molding in a shape of film, pellet or the like. Therefore, compaction or filtration or other like techniques are preferably available for producing the proton conductor.
According to the first electrochemical device of the present invention, since the proton conductor is held between the first and second electrodes, the first electrochemical device can eliminate the need for a humidifier and the like which are necessary for known fuel cells that require moisture as a migration medium so as to enhance proton conductivity. Therefore, the device construction of the present invention has an advantageously smaller and more simplified construction.
The present invention also provides a second proton conductor that includes a polymer material in addition to the fullerene derivative as previously discussed.
The present invention also provides a second method of producing a proton conductor, including the steps of: producing a fullerene derivative by introducing functional groups as discussed above; and mixing the fullerene derivative with a polymer material and forming the mixture into a desired shape, such as, a thin film.
The present invention also provides a second electrochemical device including: a first electrode, a second electrode, and a proton conductor held between the electrodes, wherein the proton conductor mainly contains a fullerene derivative as previously discussed, and a polymer material.
According to the second proton conductor of the present invention, since the conductor contains the fullerene derivative and a polymer material, it can exhibit not only an effect (high proton conductivity) comparable to that of the first proton conductor, but also a film formation ability unlike the first proton conductor that only contains the fullerene derivative. The second proton conductor, thus, can be effectively used as a thin film having a high strength, a gas permeation preventive ability, and a high proton conductivity.
According to the second production method of the present invention, since the method includes the steps of: producing a fullerene derivative, and mixing the fullerene derivative with a polymer material thereby molding the mixture into a desired shape, such as, a thin film, it can efficiently produce the proton conductor having the above unique performance in the form of a thin film. The term xe2x80x9cmoldingxe2x80x9d means molding in a desired shape by squeezing-out, compacting, coating or the like as further detailed above.
According to the second electrochemical device of the present invention, since the proton conductor that contains the fullerene derivative is held between the first and second electrodes, the second electrochemical device can exhibit an effect comparable to that of the first electrochemical device, since the proton conductor also contains the polymer material, the second electrochemical device can exhibit the same desirable effects as the second proton conductor.
The present invention also provides a third proton conductor that includes a carbon cluster derivative which has a number of functional groups so as to be capable of transferring protons between the functional groups of the carbon cluster derivative. The carbon cluster derivative includes a cluster or a plurality of clusters as its base material. The clusters each mainly or substantially contain a number of carbon atoms, preferably, on order of several to several hundred carbon atoms.
The present invention also provides a third method of producing a proton conductor, including the steps of: producing clusters of carbon atoms by an arc discharge method using a carbon-based electrode; and subjecting the carbon powder of the clusters to acid treatment or the like, to introduce functional groups to the carbon powder so as to form the carbon cluster derivative that is capable of transferring protons as previously discussed.
The present invention also provides a third electrochemical device including: a first electrode, a second electrode, and a proton conductor held between the electrodes, wherein the proton conductor mainly contains a carbon cluster derivative obtained by introducing functional groups to a cluster or a number of clusters that are the base material of the carbon cluster derivative as discussed.
The present invention has uniquely discovered that it is required to form proton conductive paths (migration sites or channels) in the carbonaceous material of the proton conductor as much as possible for imparting a good proton conductivity to the proton conductor. To meet such a requirement, it is necessary to introduce two or more functional groups that are capable of transferring protons, for example, on a surface of each of the clusters or a number of clusters, such as, a number of carbon clusters of the carbon cluster derivative. The carbon cluster is preferably made as small as possible. In this way, the carbon clusters can exhibit a desirable proton conductivity when combined in bulk to form the carbon cluster derivative of the proton conductor.
The cluster of the present invention generally means an aggregate in which atoms on order of several hundred are bonded or aggregated to each other. The aggregate improves the proton conductivity and also ensures a sufficient film strength while maintaining its chemical property to be thereby easily formed into a layer. The xe2x80x9ccluster mainly or substantially containing carbon atomsxe2x80x9d means an aggregate in which a number of carbon atoms, preferably on order of several hundred, are closely bonded to each other irrespective of the typically known molecular bonding that occurs between carbon atoms. Although this type of cluster contains a large number of carbon atoms, it is not limited only to carbon atoms and may include a variety of other atoms within its aggregate structure. Hereinafter, a cluster aggregate that contains a large number of carbon atomsxe2x80x94yet may also contain other atomsxe2x80x94is referred to as a xe2x80x9ccarbon clusterxe2x80x9d.
According to the third proton conductor of the present invention, the conductor mainly contains a carbon cluster derivative that has a chemical structure which allows protons to be easily transferred as discussed, even in a dry state, with a result that the third proton conductor can exhibit effects, such as, a desirable proton conductivity, which are similar to those of the first proton conductor. In addition, the carbon cluster derivative may include clusters or carbon clusters that contain a variety of different carbonaceous materialsxe2x80x94examples of which are discussed below.
According to the third production method of the present invention, since the production method produces the clusters or carbon clusters by making use of the arc discharge method using a carbon based electrode and subjects the carbon clusters or clusters to at least acid treatment, it can efficiently produce the carbon cluster derivative of the proton conductor having the above-described unique properties at a low cost.
According to the third electrochemical device of the present invention, since the above proton conductor is held between the first and second electrodes, the third electrochemical device can exhibit effects similar to those of the first electrochemical device.
The present invention also provides a fourth proton conductor mainly containing a tubular carbonaceous material derivative that includes functional groups so as to be capable of transferring protons between the functional groups of the tubular carbonaceous material derivative in a similar fashion as protons are transferred on the proton conductor of the previously discussed embodiments, namely the first, second, and third proton conductors, production methods, and electrochemical devices thereof.
The present invention also provides a fourth method of producing a proton conductor that includes the steps of preparing a halogenated or non-halogenated tubular carbonaceous material as a raw material; and introducing functional groups onto the tubular carbonaceous material by subjecting the material to hydrolysis and/or acid treatment or plasma treatment so as to form the tubular carbonaceous material derivative.
The present invention also provides a fourth electrochemical device that includes a first electrode, a second electrode and a proton conductor that is positioned between the electrodes wherein the proton conductor mainly contains the tubular carbonaceous material derivative as previously discussed.
The tubular carbonaceous material derivative of the fourth embodiments exhibits similar desirable and advantageous properties as the proton conductor materials of the previously discussed embodiments, such as, these materials provide a medium through which protons migrate easily even under a dry state.
As previously discussed, the principal reason why the proton conductors of the present invention can exhibit such an excellent proton migration characteristic is that a large number of functional groups, such as, hydroxyl and xe2x80x94OSO3H groups, can be introduced to the tubular carbonaceous material of the tubular carbonaceous material derivatives.
The tubular carbonaceous material derivative of the fourth embodiment includes a carbon nano-tube (CNT) material, such as, a single wall carbon nano-tube material (SWCNT), a multi-wall carbon nano-tube material (MWCNT), a carbon nano-fiber material (CNF), or other like tubular carbonaceous material.
The tubular carbonaceous material is characterized in that a ratio of an axial length to a diameter of the tubular carbonaceous material is very large, and further the tubular carbonaceous molecules of this material are entangled in a complicated form or structure that is inherent to this kind of material. Accordingly, a large number of the functional groups can be introduced onto the surfaces of the tubular carbonaceous molecules of these carbonaceous materials (see FIGS. 20-22).
In particular, the tubular carbonaceous material of the fourth embodiment makes it possible to increase the number of stable proton sites from which the protons can singly migrate without the use of carrier molecules, such as, water, and to continuously distribute the stable proton sites over an entire region of the material.
The fourth method of producing a proton conductor that includes a tubular carbonaceous material derivative as discussed can be easily produced by preparing a halogenated or non-halogenated tubular carbonaceous material; then subjecting the material to an acid treatment or hydrolysis and an acid treatment or a plasma treatment. This material can then be easily formed into a film by dispersing the tubular carbonaceous material derivative within a liquid such as water and then filtering the dispersion of the derivative.
The film thus formed, in which tubular molecules are entangled, has a large strength, a high stability, and a good proton conductivity. When used for a general electrochemical device, the proton conductor is required to be configured as an aggregate of the tubular carbonaceous material derivative, and in particular, when used for a fuel cell, the proton conductor is required to be configured as a thin film having a high stability, a high density, a large strength, and a good proton conductivity. Accordingly, it is apparent that the film of the tubular carbonaceous material derivative according to the present invention is particularly suitable for such an application.
The film can then be used for an electrochemical device wherein the proton conductor of the electrochemical device is formed of the film. In this way, the film is mounted as the proton conductor between the first and second electrodes of the electrochemical device such that it is possible to maintain desirable proton conductivity for a long period of time without the need of using any external migration medium, such as, moisture so as to enhance proton conductivity.